Selection of Cu2+ for intercalation from the electronegativity perspective: Improving the cycle stability and rate performance of δ-MnO2 cathode material for aqueous zinc-ion batteries

Ion intercalation is an effective strategy for improving the cycle stability and rate performance of δ-MnO2 as a cathode material for aqueous zinc-ion batteries. However, in practice, ion selection appears rather arbitrary. In this work, Cu2+ was chosen for δ-MnO2 intercalation because although Cu2+ and Zn2+ have similar diameters, Cu2+ has a slightly higher electronegativity (1.359) than Zn2+ (1.347). Therefore, Cu2+ has a stronger interaction with the MnO2 lattice than Zn2+ and can be stable during the intercalation/deintercalation of Zn2+ and H+. Results showed that the performance of Cu-doped δ-MnO2 (CMO) was greatly improved. Moreover, at the high current density of 2 A g−1, CMO achieved excellent cycle stability with 100% capacity retention after 600 cycles, whereas pristine δ-MnO2 exhibited only 23% capacity retention. When the current density was increased from 0.2 to 2.0 A g−1, the CMO electrode also delivered remarkable rate performance with 72% capacity retention, which was considerably higher than the 32% capacity retention demonstrated by pristine δ-MnO2. Given that Cu2+ has a greater electronegativity than Zn2+, the Cu-O bond formed in CMO acted as a stable structural column and greatly improved the stability of CMO. Cu2+ doping also increased the electronic conductivity and ionic conductivity of CMO and reduced the charge transfer resistance of H+ and Zn2+ at the electrode/electrolyte interface, which improved the rate performance of CMO greatly. This work provides new insights into intercalation strategies to improve the electrochemical performance of batteries.